1. Field of the Invention
The present invention relates to a magnetic recording medium such as a magnetic tape or magnetic disc. More particularly, it relates to a magnetic recording medium which comprises a non-magnetic substrate and a magnetic layer formed thereon containing magnetic powder and a binder.
2. Description of the Prior Art
Conventional magnetic recording mediums have a magnetic layer which is generally prepared by coating on a non-magnetic substrate, such as a polyester film, a magnetic paint containing magnetic powder and a binder and then drying it. Electromagnetic conversion properties of the magnetic recording medium are generally defined by saturaion magnetic flux density (Bs) and the surface smoothness of the magnetic layer. The higher the saturation magnetic flux density and surface smoothness, the higher output in recording and reproduction. Therefore, various attempts to improve such properties have been made to satisfy a recent requirement of enhancing the functions of the magnetic recording medium.
Conventionally it is believed that it is most effective to use a magnetic powder contained in the magnetic layer having high saturation magnetization (.sigma..sub.s) in order to enhance the saturation magnetic flux density, and it is particularly effective to use fine magnetic powder having a small average particle size, i.e. a large BET specific surface area, in order to improve the surface smoothness. In view of the above, it is believed that among the conventional magnetic powders, powder of magnetic metal oxide such as .gamma.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4 is not suitable since it leads to low saturation magnetization and unsatisfactory saturation magnetic flux density of the magnetic recording medium even if the powder is fine, while magnetic metallic powder having a high saturation magnetization and a small average particle size, such as iron powder, is suitable.
However, such a magnetic metallic powder has high coercive force (Hc) and it is very difficult to control the coercive force to the value of not more than 1,000 Oe, and the finer the powder is, the higher the coercive force of the magnetic recording medium is. Therefore, as a magnetic head used for recording and reproduction of the magnetic recording medium, an expensive metallic head such as an amorphous or Sundust head should be used. But, a widely used ferrite head can not lead to sufficient properties. To achieve durability required for the magnetic recording medium in addition to the electromagnetic conversion properties, the magnetic recording medium utilizing the magnetic metallic powder is much worse than one utilizing the magnetic oxide powder.
A magnetic recording medium which utilizes metallic powder having a small BET specific surface area, i.e. the large average particle size, can have a coercive force of not more than 1,000 Oe, but the magnetic layer has a much worse surface smoothness as described above. A magnetic recording medium utilizing magnetic oxide powder can have a controlled coercive force of not more than 1,000 Oe without deteriorating the surface smoothness and has good durability, but it cannot achieve high saturation magnetic flux density as described above.